A Large Population of Laboratory Animals Has Been Allowed to Breed

Multiple Choice

A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (A2A2) , the same percentage as at the beginning of the breeding program. The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants.
What is the most reasonable conclusion that can be drawn from the fact that the frequency of the recessive trait (A2A2) has not changed over time?

A) The two phenotypes are about equally adaptive under laboratory conditions.
B) The genotype A1A1 is lethal.
C) There has been a high rate of mutation of allele A1 to allele A2.
D) There has been sexual selection favoring allele A2.